Method of selectively etching silicon dioxide dielectric layers on semiconductor wafers

ABSTRACT

A method of utilizing and etching SiO 2  in the processing of semiconductor wafers comprises: a) providing a layer of undoped SiO 2  atop a wafer; b) providing a layer of doped SiO 2  atop the layer of undoped SiO 2  ; and c) wet etching the layer of doped SiO 2  selectively relative to the undoped layer of SiO 2  utilizing an acid solution, the acid solution comprising a mixture of at least two different mineral acids provided in a selected ratio relative to one another, one of the mineral acids being HF. The preferred volumetric ratio of other mineral acids in the acid solution to HF in the acid solution is from 20:1 to 110:1, with a ratio of from 45:1 to 65:1 being most preferred. Example acids to be combined with the HF include H 2  SO 4 , HCl, HNO 3 , H 3  PO 4 , HBr, HI, HClO 4 , and HIO 4 , or mixtures thereof.

TECHNICAL FIELD

This invention relates generally to methods of utilizing and etchingSiO₂ in the processing of semiconductor wafers.

BACKGROUND OF THE INVENTION

At some point in the processing of semiconductor wafers, it is common toneed to etch one dielectric layer selectively relative to an underlyingdielectric layer. This is typically accommodated by utilizing chemicallydifferent dielectric layers, such as an oxide and a nitride, andutilizing a wet acid or other chemistry which enables etching of theoverlying material relative to the underlying material. One drawbackwith such techniques is that typically the two different dielectrics donot have the same thermal coefficient of expansion. This can createundesired stress in the resulting structure where remaining adjoinedlayers of an oxide and a nitride exist.

Silicon dioxide can be provided atop a wafer in both doped and undopedforms. Example dopants include phosphorus and boron, which can beutilized to produce phosphosilicate glass (PSG), borosilicate glass(BSG) and borophosphosilicate glass (BPSG). Such dopants can improvestep coverage, diffusion barrier properties, stress reduction, andimprove flow characteristics enabling dielectric flow at lowertemperatures. Additionally, doped and undoped silicon dioxide has verysimilar thermal expansion coefficients.

It would be desirable to develop a wet etching chemistry that wouldenable etching of doped silicon dioxide selectively relative to undopedsilicon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic section of a semiconductor wafer at aprocessing step in accordance with the invention.

FIG. 2 is a diagrammatic section of the FIG. 1 wafer illustrated at aprocessing step subsequent to that shown by FIG. 1.

FIG. 3 is a diagrammatic section of the FIG. 1 wafer illustrated at aprocessing step subsequent to that shown by FIG. 2.

FIG. 4 is a diagrammatic section of the FIG. 1 wafer illustrated at aprocessing step subsequent to that shown by FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

In accordance with the invention, a method of utilizing and etching SiO₂in the processing of semiconductor wafers comprises the following steps:

providing a layer of undoped SiO₂ atop a wafer;

providing a layer of doped SiO₂ atop the layer of undoped SiO₂ ; and

wet etching the layer of doped SiO₂ selectively relative to the undopedlayer of SiO₂ utilizing an acid solution, the acid solution comprising amixture of at least two different mineral acids provided in a selectedratio relative to one another, one of the mineral acids being HF.

The undoped SiO₂ can be deposited or provided by any known technique.The doped SiO₂ can be any of borosilicate glass, phosphosilicate glass,borophosphosilicate glass, or others. The invention has been reduced topractice utilizing undoped SiO₂ deposited by decomposition oftetraethylorthosilicate (TEOS) followed by TEOS densification, with thedoped SiO₂ comprising borophosphosilicate glass having six weightpercent phosphorus and three weight percent boron.

It is expected that the other of the mineral acids could comprise anyvariety of mineral acid or combinations of mineral acids. In the contextof this document, "mineral acid" has its standard definition of aninorganic acid. The invention has been reduced to practice utilizingother mineral acids of H₂ SO₄, HCl and HNO₃. HNO₃, however, provides adisadvantage of not providing selectivity relative to silicon during theetch. Accordingly where silicon is or will be exposed by etch of thedoped oxide, HNO₃ would typically not be an acid of choice. Exampleother preferred mineral acids include H₃ PO₄, HBr, HI, HClO₄, and HIO₄,or mixtures thereof.

The preferred volumetric ratio of other mineral acids in the acidsolution to HF in the acid solution is from 20:1 to 110:1, with 45:1 to65:1 being most preferred. The invention was reduced to practiceutilizing a ratio of 55:1. For ratios toward the 110:1 point,selectivity is expected to be high, while etch rate is expected to below. At a ratios toward the 20:1 point, etch rate is expected to behigh, while selectivity will be reduced.

The preferred solution temperatures for etching are expected to fallbetween 20° C. and 50° C., with about 35° C. being most preferred. Lowerbath temperatures are expected to provide better selectivity, and aslower etch rate, while higher bath temperatures are expected to providepoorer selectivity and a higher etch rate.

An example etch solution with which the invention was reduced topractice comprised 840 ml H₂ SO₄, 960 ml H₂ O, 30 ml HF, and 60 mlacetic acid. The H₂ SO₄ solution was comprised of 97% H₂ SO₄. The HFsolution comprised 49% by volume HF in H₂ O. This thus provided in theetching solution a volumetric ratio of H₂ SO₄ :HF of 55:1. Acetic acidis a known prior art component in wet acid etching which provides bathstabilization and extends bath life. Such does not constitute a part ofthis invention, nor is it necessary in the invention. Bath temperaturewas maintained at 35° C. during etching. Such chemistry was determinedto etch borophosphosilicate glass (six weight percent phosphorus, threeweight percent boron) at a rate of 108 Angstroms per second, whileetching densified TEOS at a rate of 17 Angstroms per second, thusproviding a selectivity ratio of 6.3 to 1. Corresponding etch rates weredetermined utilizing HCl and HNO₃ in place of H₂ SO₄.

An example implementation utilizing the above described technology isshown and described with reference to FIGS. 1-4 for formation of cellcapacitor storage nodes. FIG. 1 illustrates a semiconductor waferfragment 10 having a pair of word lines 12, 14 and a field oxide region16 provided over a bulk substrate 18. Field oxide 16 would compriseundoped SiO₂. Word lines 12, 14 are as well shown surrounded by undopedSiO₂, provided in the form of spacers 20 and caps 22. Such wouldtypically be deposited by decomposition of TEOS, followed by well knowndensification techniques.

Referring to FIG. 2, a layer 24 of BPSG is applied.

Referring to FIG. 3, a layer 26 of photoresist is applied, patterned anda dry etch conducted through BPSG layer 24 to open a desired capacitorcontact 28 to an active area (not shown) in substrate 18.

Referring to FIG. 4, the wafer would then be dipped in a wet acidsolution in accordance with the invention, with photoresist 26 still inplace, to effect an isotropic etch of the BPSG layer. Such which will beselective to the undoped TEOS SiO₂ and field oxide, and photoresist.Thereafter, the photoresist layer is removed, producing the FIG. 4construction. The illustrated and enlarged cavity 30 which is producedcan be utilized for a cell storage node capacitor.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

We claim:
 1. A method of utilizing and etching SiO₂ in the processing ofsemiconductor wafers, the method comprising the followingsteps:providing a layer of undoped SiO₂ atop a wafer; providing a layerof doped SiO₂ atop the layer of undoped SiO₂ ; and wet etching the layerof doped SiO₂ selectively relative to the undoped layer of SiO₂utilizing an acid solution, the acid solution comprising a mixture of atleast two different mineral acids provided in a selected ratio relativeto one another, one of the mineral acids being HF, the other of themineral acids being selected from the group consisting of H₂ SO₄, H₃PO₄, HBr, HI, HClO₄, and HIO₄, or mixtures thereof.
 2. The method ofutilizing and etching SiO₂ of claim 1 wherein the volumetric ratio ofother mineral acids in the acid solution to HF in the acid solution isfrom 20:1 to 110:1.
 3. The method of utilizing and etching SiO₂ of claim1 wherein the volumetric ratio of other mineral acids in the acidsolution to HF in the acid solution is from 45:1 to 65:1.
 4. The methodof utilizing and etching SiO₂ of claim 1 wherein the volumetric ratio ofother mineral acids in the acid solution to HF in the acid solution isabout 55:1.
 5. The method of utilizing and etching SiO₂ of claim 1wherein the doped layer of SiO₂ comprises borophosphosilicate glass. 6.The method of utilizing and etching SiO₂ of claim 1 wherein,thevolumetric ratio of other mineral acids in the acid solution to HF inthe acid solution is from 20:1 to 110:1; and the doped layer of SiO₂comprises borophosphosilicate glass.
 7. The method of utilizing andetching SiO₂ of claim 1 wherein,the volumetric ratio of other mineralacids in the acid solution to HF in the acid solution is from 45:1 to65:1; and the doped layer of SiO₂ comprises borophosphosilicate glass.8. A method of utilizing and etching SiO₂ in the processing ofsemiconductor wafers, the method comprising the followingsteps:providing a layer of undoped SiO₂ atop a wafer; providing a layerof doped SiO₂ atop the layer of undoped SiO₂ ; and wet etching the layerof doped SiO₂ selectively relative to the undoped layer of SiO₂ andselectively relative to silicon utilizing an acid solution, the acidsolution comprising a mixture of at least two different mineral acidsprovided in a selected ratio relative to one another, one of the mineralacids being HF, the other of the mineral acids being H₂ SO₄.
 9. Themethod of utilizing and etching SiO₂ of claim 8 wherein the volumetricratio of H₂ SO₄ in the acid solution to HF in the acid solution is from20:1 to 110:1.
 10. The method of utilizing and etching SiO₂ of claim 8wherein the volumetric ratio of H₂ SO₄ in the acid solution to HF in theacid solution is from 45:1 to 65:1.
 11. The method of utilizing andetching SiO₂ of claim 8 wherein the volumetric ratio of H₂ SO₄ in theacid solution to HF in the acid solution is about 55:1.
 12. The methodof utilizing and etching SiO₂ of claim 8 wherein the doped layer of SiO₂comprises borophosphosilicate glass.
 13. The method of utilizing andetching SiO₂ of claim 8 wherein,the volumetric ratio of H₂ SO₄ in theacid solution to HF in the acid solution is from 20:1 to 110:1; and thedoped layer of SiO₂ comprises borophosphosilicate glass.
 14. The methodof utilizing and etching SiO₂ of claim 8 wherein,the volumetric ratio ofH₂ SO₄ in the acid solution of HF in the acid solution is from 45:1 to65:1; and the doped layer of SiO₂ comprises borophosphosilicate glass.15. The method of utilizing and etching SiO₂ of claim 1 wherein the acidsolution consists essentially of HF and H₂ SO₄.
 16. The method ofutilizing and etching SiO₂ of claim 1 wherein the acid solution consistsessentially of HF and H₃ PO₄.
 17. The method of utilizing and etchingSiO₂ of claim 1 wherein the acid solution consists essentially of HF andHBr.
 18. The method of utilizing and etching SiO₂ of claim 1 wherein theacid solution consists essentially of HF and HI.
 19. The method ofutilizing and etching SiO₂ of claim 1 wherein the acid solution consistsessentially of HF and HClO₄.
 20. The method of utilizing and etchingSiO₂ of claim 1 wherein the acid solution consists essentially of HF andHIO₄.